The present invention relates to an emergency disconnector for disconnecting a fluid loading and unloading line in an emergency to cut off the flow of a fluid of an extremely low temperature, such as LNG (liquified natural gas), being conveyed through the line.
Fluid loading and unloading lines are used in such applications as for unloading LNG from an LNG tanker alongside a sea berth to a storage tank or loading LNG from the storage tank to the LNG tanker.
Under rough weather conditions, the LNG tanker tends to become detached from the sea berth due to strong winds or high waves while loading or unloading LNG. In such an emergency, it is necessary that the couplings in the fluid loading and unloading line be disconnected to sever a loader/unloader on the sea berth from the LNG tanker and to cut off the flow of dangerous LNG to prevent unwanted leakage. To meet such a requirement, the fluid disconnector for disconnecting the line in an emergency must quickly stop the fluid flow.
FIGS. 8 through 10 of the accompanying drawings illustrate an emergency line disconnector as proposed in U.S. patent application Ser. No. 444,115, filed concurrently with the present application. This line disconnector comprises a pair of couplings C.sub.1, C.sub.2 interconnected by connectors B and having therein a pair of slide valves V.sub.1, V.sub.2, respectively. The slide valves V.sub.1, V.sub.2 have a pair of projecting spacer rods L.sub.1, L.sub.2. respectively, held against each other to displace the slide valves in opposite directions against the biasing forces of springs S.sub.1, S.sub.2 to define a space between the slide valves V.sub.1, V.sub.2, thereby providing a fluid passageway between the couplings C.sub.1, C.sub.2 and hence the slide valves V.sub.1, V.sub.2. A pair of seal packings P, P of nitrile rubber are respectively secured to rings R, R by curing, and are attached to heads of slide valves V.sub.1, V.sub.2 by screws B, as illustrated in FIG. 9.
During normal operation in conveying a fluid, the fluid flows from the coupling C.sub.2 through slots SL.sub.2 in the slide valve V.sub.2 and slots SL1 in the slide valve V.sub.1 into the coupling C.sub.1, from which the fluid is discharged, as shown in FIG. 8. In an emergency requiring fluid cutoff, the connectors B, B are removed by external forces to allow the couplings C.sub.1, C.sub.2 to be disconnected, whereupon the seal packings P, P on the slide valves V.sub.1, V.sub.2 are moved under the biasing forces of the springs S.sub.1, S.sub.2 into abutment against sealing portions r.sub.1, r.sub.2 of the couplings C.sub.1, C.sub.2, thus cutting off the flow of the fluid as shown in FIG. 10. The seal packings P, P of the emergency line disconnector as described above are required to meet the following requirements:
(1) The material of the seal packing itself should be resilient; and
(2) The packing should be secured to the metal rings by heat treatment or the like.
With such an emergency line disconnector, seal packings P, P of rubber, which become brittle at -30.degree. or below cannot withstand sustained use in transporting a fluid such as LNG having a temperature of -196.degree. or lower, that is a cryogenic temperature. A desired degree of sealing cannot be assured between the sealing portions r.sub.1, r.sub.2 and the slide valves V.sub.1, V.sub.2, resulting in a tendency for the fluid to leak from the couplings C.sub.1, C.sub.2 upon emergency line disconnection, a condition which could give rise to an accident.
The seal packings, P, P of rubber could be replaced with seal packings of Teflon mounted on the heads of the slide valves V.sub.1, V.sub.2, seal packings of Teflon being free from brittleness at extremely low temperature. However, seal packings of Teflon would be likely to come off due to dimensional differences between the metal heads and the seal packings, which would shrink differently at extremely low temperatures. Additionally, seal packings of Teflon are much less resilient than seal packings of rubber, so that the sealing portions r.sub.1, r.sub.2 could not be completely sealed by the slide valves V.sub.1, V.sub.2 even with an increased pressure produced by the springs S.sub.1, S.sub.2.
In addition, since the couplings C.sub.1, C.sub.2 of the above-desired emergency disconnector are sealed off in a very short period of time, after having been separated, by the resilient force of the springs S.sub.1, S.sub.2 and fluid pressure, the kinetic energy of the fluid flowing through the line is converted into a pressure upon sudden cutoff of the fluid flow, resulting in problems such as water hammer.
In order to employ this emergency disconnector, it has often been necessary to take suitable additional measures, such as the provision of an additional check valve.